The majority of motorcycles and bicycles utilize a drive train with a conventional chain transmitting power from a first pulley to a second pulley associated with a hub of a rear wheel. Chains are relatively effective at transmitting power between the first and second pulleys, but they are not without shortcomings. The shortcomings include the need for periodic lubrication, loss of efficiency as they wear, and chains can be noisy.
One known alternative to chain-based systems are belt drive systems. Belt drive systems overcome a number of the problems discussed above, but are not without their own problems. For example, belt drive systems generally require relatively precise alignment between first and second sprockets of the belt drive system. Providing such alignment can be difficult particularly in bicycles, where crank sets and hubs of various manufacturers can be mixed and matched in bicycle assembly, resulting in less than precise alignment. In addition, incidents may happen during use of the bike that degrades the sprocket alignment. One known way of addressing this misalignment is to provide radially extending side flanges on each sprocket to prevent the belt from wandering off the sprocket. However, such side flanges can complicate the manufacture of the sprocket, increase weight, and make the pulley wider, all of which can increase the cost of the sprocket. A further problem with known belt drive systems is collection of debris between teeth of the sprockets which can interfere with effective force transmission and, in extreme cases, cause a belt to break or jump off a sprocket.
Other systems are known which employ a single flange located in the center of the sprocket. The flange extends about the entire circumference. The cooperating belt comprises a central groove which the flange engages thereby ensuring proper alignment during operation. The flange extends between adjacent sprocket teeth. The flange prevents lateral movement and eventual disengagement of the belt from the sprocket. However, the central groove can accumulate debris which then interferes with operation. The flange also represents added weight for the sprocket.
Each of these disadvantages is significant for cycle applications and particularly bicycle applications.
Representative of the art is U.S. Pat. No. 8,136,827 which discloses a self-aligning belt drive system comprises a belt drive having a plurality of longitudinally spaced inner lobes each having an alignment groove. The system further comprises at least one pulley comprising a frame configured to rotate about a rotation axis and having a circular outer rim. A plurality of circumferential teeth extend radially and axially of the rim with each tooth being configured to be received between adjacent inner lugs of the drive belt. An alignment flange extends radially between circumferential teeth. The alignment flange is configured to be received in the alignment groove and the alignment flange extends no further radially from the rotational axis than the circumferential teeth. The circumferential teeth, the alignment flange and the alignment groove are configured so that with the alignment flange received in the alignment groove, the drive belt rides on the circumferential teeth.
What is needed is a sprocket having a planar fin extending from one side of a sprocket tooth, the planar fin configured to engage the longitudinal belt groove, the planar fin extending into a sprocket groove between adjacent sprocket teeth not greater than 20% of a groove width (W). The present invention meets this need.